Acholeplasma laidlawii (A. laidlawii; ATCC 23206) is a wall-less bacterium in the Mollicutes class. Contrary to Mycoplasma, which belong to a different taxonomic order and have high sterol content membranes and thus require sterol addition to the growth media, A. laidlawii does not require sterols. Acholeplasma laidlawii has many fatty acid modified membrane proteins. Since it cannot make the unsaturated fatty acids it needs, it has a requirement for branched chain fatty acids in the growth media. Acholeplasma laidlawii makes acid from carbohydrates and lowers the pH of the growth media. However, it is sensitive to low pH.
Owing to its small size and common contaminant status, A. laidlawii is used as a model mycoplasma for retention testing 0.1 μm rated sterilizing grade filters. Industry and most laboratories use (horse) serum-containing media to cultivate it. Many media variants exist, as there currently is no standard medium or retention test recommended by federal or industry regulators for such filters. The test format is based on the existing Brevundimonas diminuta (B. diminuta) test for 0.2 μm rated sterilizing grade filters. Thus far, there are no standards for the media and methods used to grow A. laidlawii. Without such standards, it is difficult to compare and evaluate the retention data, especially since the media components affect the organism size and number, which, in turn, affect retention capacity, in terms of logarithmic reduction value (LRV).